Hybrid reactive power compensation device

ABSTRACT

A hybrid reactive power compensation device comprises a passive type reactive power compensator and an active type reactive power compensator serially connected thereto. The passive type reactive power compensator is an AC power capacitor adapted to provide the reactive power that reduces capacity of the active type reactive power compensator. The active type reactive power compensator is consisted of a power converter, a DC capacitor, a high-frequency ripple filter and a controller. The hybrid reactive power compensation device can supply a linearly adjustable reactive power within a predetermined range, and the supplied current is approximated to be a sinusoidal waveform. Therefore, it can avoid the destruction of AC power capacitor caused by the power resonance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to a hybrid reactive powercompensation device including a passive type reactive power compensatorand an active type reactive power compensator serially connectedthereto, which are adapted to supply a linearly adjustable reactivepower within a predetermined range in the distribution power system. Thepresent invention is related to a hybrid reactive power compensationdevice including an active type reactive power compensator adapted toadjust a current flowing through the passive type reactive powercompensator to be approximated as a sinusoidal waveform, and thereby itcan avoid the power resonance generated between itself and the reactanceof power system that may destroy the reactive power compensation deviceand adjacent power facilities.

[0003] 2. Description of the Related Art

[0004] Most of loads in distribution power system have thecharacteristic of inductance, and it will result in the poor powerfactor. Hence, it requires a larger current for the identical real powerthat reduces the power efficiency of distribution power system anddegrades the performance of voltage regulation of the load side. Forsolving the above problems, power substations and power consumersgenerally install a passive type reactive power compensator (AC powercapacitors) parallel connected to the distribution power system, so asto compensate a lagging reactive power to increase the entire powerfactor. In some distribution power system, the capacity of applied ACpower capacitor is about 25% to 35% of total capacity, and in some otherdistribution power system even exceeds about 50%, according to researchreports.

[0005] Recently, harmonic pollution of industrial power system isincreased seriously due to the wide use of nonlinear loads. The AC powercapacitor for power factor correction provides with a low impedance pathfor harmonic current, hence, the AC power capacitor is frequentlydamaged by harmonics. Meanwhile, it results in the power resonancebetween the AC power capacitor and the distribution power system. Then,it will result in the amplification of harmonic current and harmonicvoltage. Thus, the destruction of the AC power capacitor due toover-voltage or over-current may occur. Besides, the over-voltage of ACpower capacitor caused by the power resonance may destroy neighboringelectric power facilities and even result in public accidents.

[0006] In order to solve the power resonance problem caused by the ACpower capacitor, the voltage rating is increased to avoid thedestruction of over-voltage in conventional solution. However, it cannotresolve the resonance problem and may, therefore, cause the destructionof neighboring power facilities.

[0007] There is another solution that the AC power capacitor is switchedoff from the power system when over-voltage or over-current occurs, butthe function of reactive power compensation will be failed.

[0008] The reactive power compensation also can be obtained by using aset of constant AC power capacitors merely providing a fixed reactivepower. This fixed reactive power cannot be adjusted to respond to thevariation of loads, and it may result in over-voltage due to the lightload. In order to properly adjust reactive power provided by the ACpower capacitor, an automatic power factor regulator (APFR) isdeveloped, as shown in FIG. 1. The APFR is consisted of a set of ACpower capacitors C₁ through C_(N) via switches S₁ through S_(N). Therebythe reactive power supplied from the APFR can be adjusted by changingnumber of AC power capacitors switching on. Although APFR can supply anadjustable reactive power to respond to the variation of loads, the APFRcan merely be adjusted step by step not linearly. Therefore, the powerfactor of the distribution power system compensated by APFR still cannotbe close unity.

[0009] Referring to FIG. 2, another power factor regulator uses a fixedcapacitor parallel connected to a controllable reactor 11, which iscontrolled by a thyristor switch 10. This power factor regulator,so-called a Fixed-Capacitor Thyristor-Controlled Reactor (FC-TCR), usesphase control technique to control the thyristor switch 10, thereby itcan provide with a linearly adjustable reactive power. However, itgenerates a significant amount of harmonic current and results inserious harmonic pollution due to the use of phase control technique inthyristor.

[0010] The reactive power is adjustable in the two reactive powercompensation devices described in preceding paragraphs, but the AC powercapacitor thereof is parallel connected to a power system and it stillcannot avoid the problem of destruction caused by the power resonance.

[0011] Referring to FIG. 3, it illustrates a facility based on powerelectronic technology to be applied in a distribution power system tocompensate reactive power, so-called the active type reactive powercompensator 2. This active type reactive power compensator uses a powerconverter 20 via an inductor 21 to be connected to a power system 1. Thepower converter 20 is connected to a DC power capacitor 22 at its DCside. The active type reactive power compensator 2 may provide with aleading reactive power or a lagging reactive power. The suppliedreactive power can be adjusted linearly to respond to the variation ofloads that the input power factor can be maintained to be close tounity. Meanwhile, the active power factor correction system will notresult in power resonance. Hence, it can avoid the destruction of thepower resonance generated by an AC power capacitor. However, the activetype reactive power compensator 2 must compensate the reactive powerrequired by the loads, it requires a large capacity of power converterin the active type reactive power compensator. Hence, the wideapplication is limited due to the high cost.

[0012] The present invention intends to provide a hybrid reactive powercompensation device used for supplying the linearly adjustable reactivepower within a predetermined range. Meanwhile, the hybrid reactive powercompensation device includes an active type reactive power compensatorto adjust the compensation current to be approximated as a sinusoidalwaveform, and thereby it can avoid the power resonance generated betweenitself and the reactance of power system. Therefore, it can avoid thedestruction of itself and the neighboring power facilities by the powerresonance. Moreover, the manufacture cost of the present invention isless expensive than that of the conventional active type reactive powercompensator.

SUMMARY OF THE INVENTION

[0013] The primary objective of this invention is to provide a hybridreactive power compensation device including a passive type reactivepower compensator and an active type reactive power compensator seriallyconnected thereto, which adapted to supply a linearly adjustablereactive power and thereby avoid the destruction of power resonance. Themanufacture cost of this invention is less expensive than that of theconventional active type reactive power compensator.

[0014] The hybrid reactive power compensation device in accordance withthe present invention mainly comprises a passive type reactive powercompensator and an active type reactive power compensator seriallyconnected thereto. The passive type reactive power compensator is an ACpower capacitor adapted to provide with reactive power that, thus,reduces reactive power supplied from the active type reactive powercompensator. Additionally, it can reduce the voltage rating and thecapacity of active type reactive power compensator. Since the cost of ACpower capacitor is less expensive significantly than that of the activetype reactive power compensator, the manufacture cost of the presentinvention is also less expensive than that of the conventional activetype reactive power compensator. The active type reactive powercompensator is consisted of a power converter, a DC capacitor, ahigh-frequency ripple filter and a controller. The hybrid reactive powercompensation device is adapted to supply linearly adjustable reactivepower within a predetermined range. The hybrid reactive powercompensation device can supply a current with a nearly sinusoidalwaveform for reactive power compensation due to the use of active typereactive power compensator, and thereby it can avoid the power resonancegenerated by itself and reactance of the power system. Therefore, it canavoid the destruction of itself and neighboring power facilities due tothe power resonance.

[0015] Other objectives, advantages and novel features of the inventionwill become more apparent from the following detailed description andthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will now be described in detail withreference to the accompanying drawings herein:

[0017]FIG. 1 is a schematic view of a conventional automatic powerfactor regulator in accordance with the prior art;

[0018]FIG. 2 is a structural schematic view of a conventionalfixed-capacitor thyristor-controlled reactor in accordance with theprior art;

[0019]FIG. 3 is a structural schematic view of a conventional activetype reactive power compensator in accordance with the prior art;

[0020]FIG. 4 is a structural schematic view of a hybrid reactive powercompensation device in accordance with a first embodiment of the presentinvention;

[0021]FIG. 5 is a control block diagram of active type reactive powercompensator in accordance with the first embodiment of the presentinvention;

[0022]FIG. 6 is a structural schematic view of a parallel connection ofa hybrid reactive power compensation device with an automatic powerfactor regulator system in accordance with a second embodiment of thepresent invention; and

[0023]FIG. 7 is a structural schematic view of a hybrid reactive powercompensation device in accordance with a third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0024]FIG. 4 illustrates a system structure of a hybrid reactive powercompensation device in accordance with the first embodiment of thepresent invention. Referring to FIG. 4, the hybrid reactive powercompensation device 3 is parallel connected between a power system 1 anda load 4. The power system 1 provides an AC power to the load 4. Thehybrid reactive power compensation device 3 is adapted to compensate thereactive power required by the load 4 to thereby improve the powerfactor from the view of power system 1. The hybrid reactive powercompensation device 3 includes a passive type reactive power compensator31 and an active type reactive power compensator 32 serially connectedthereto. The passive type reactive power compensator 31 is a powercapacitor adapted to supply the reactive power, thereby reducing thereactive power supplied from the active type reactive power compensator32. The active type reactive power compensator 32 includes a powerconverter 320, a DC power capacitor 321, a high-frequency ripple filter322 and a controller 323. The active type reactive power compensator 32is used to linearly adjust the reactive power supplied from the hybridreactive power compensation device 3 within a predetermined range. Inaddition, the active type reactive power compensator 32 can avoid thedestruction of power resonance generated between the passive typereactive power compensator 31 and the impedance of power system 1.

[0025]FIG. 5 illustrates a block diagram of the controller 323 of theactive type reactive power compensator 32 in accordance with the firstembodiment of the present invention. Referring again to FIG. 5, theactive type reactive power compensator 32 adopts the current modecontrol, which applies a power converter 320 for controlling asinusoidal current with 90 degrees leading with the voltage of powersystem. The output current of power converter is passing through thehybrid reactive power compensation device 3. Subsequently, the reactivepower supplied from the hybrid reactive power compensation device 3 canbe adjusted by controlling the amplitude of the fundamental component ofpower converter's output current. Consequently, the hybrid reactivepower compensation device 3 can avoid the destruction of power resonancebecause the current passing through is a sinusoidal waveform with thefundamental frequency.

[0026] Referring to FIGS. 4 and 5, the controller 323 provides with acurrent reference signal, which is consisted of two control signals S₁and S₂. The first control signal S₁ is used for adjusting the reactivepower. The first control signal S₁ must be leading the voltage signal offundamental component of the power system 1 by 90 degrees since theactive type reactive power compensator 32 applies the current modecontrol. The load current is sent to the first band-pass filter 500 toobtain its fundamental component, and the voltage of power system issent to the second band-pass filter 501 to obtain its fundamentalcomponent. Then, both outputs of the first band-pass filter 500 and thesecond band-pass filter 501 are fed to the reactive power calculatingcircuit 502. The reactive power calculating circuit 502 calculates andsupplies the desired amplitude of reactive power current demanded by thehybrid reactive power compensation device 3.

[0027] In order to obtain the wave-shape of the reactive power current,the fundamental component supplied from the second band-pass filter 501is sent to a phase-shift circuit 503 that may produce the signal whichphase is 90 degrees leading with the fundamental component of powersystem voltage. After that the outputs of the phase-shift circuit 503and the reactive power calculating circuit 502 are sent to a multiplier504 for obtaining the first control signal S₁. The second control signalS₂ is used to regulate the voltage of DC power capacitor 321 of theactive type reactive power compensator 32. The active type reactivepower compensator 32 has power loss and thus the voltage of DC powercapacitor 321 may be varied. In order to maintain the active typereactive power compensator 32 operated normally, the DC voltage thereofmust be maintained at a constant value. In this condition, the activetype reactive power compensator 32 must absorb/generate real powerfrom/to the power system 1. It means that the active type reactive powercompensator 32 must generate a fundamental component current which phaseis in phase with the phase of the power system 1 voltage. For obtainingthis purpose, the DC voltage 321 of the active type reactive powercompensator 32 is detected. The detected DC voltage 321 and a presetvoltage must be sent to a subtractor 505, and then the subtracted resultis sent to the first P-I controller 506. The output of the firstcontroller 506 and the output fundamental signal of the second band-passfilter 501 are sent to a multiplier 507 to get second control signal S₂.

[0028] Referring to FIGS. 4 and 5, the reference signals can be obtainedwhen the two control signals S₁ and S₂ are added in an adder 508. Thenthe reference signals and the output current of the active type reactivepower compensator 32 are sent to a subtractor 509. The output of thesubtractor 509 is passed to the second P-I controller 510 to obtain amodulation signal, and then the modulation signal is sent to apulse-width modulation circuit 511 to generate the pulse-widthmodulation signal. Consequently, the pulse-width modulation signal issent to a driver circuit 512. Then, the driving signals of the powerconverter 320 of the active type reactive power compensator 32 can beobtained.

[0029] Referring to FIG. 6, it is illustrated that the second embodimentincludes the hybrid reactive power compensation device 3 of the firstembodiment and an automatic power factor regulator system (APFR system)6 connected parallel thereto. The connected hybrid reactive powercompensation device 3 and APFR system 6 is parallel connected betweenthe power system 1 and the load 4. The power system 1 supplies the ACpower to the load 4. The combination of the hybrid reactive powercompensation device 3 and the APFR system 6 is used to supply thereactive power for compensating the reactive power demanded by the load4. The APFR system 6 adjusts the reactive power step by step for roughtuning, and the hybrid reactive power compensation device 3 adjusts thereactive power linearly for fine tuning so that improves the input powerfactor to be closed to unity. Thus the capacity of the hybrid reactivepower compensation device 3 is reduced. Consequently, the secondembodiment merely requires a relatively small capacity of the hybridreactive power compensation device 3 to incorporate into the APFR system6 and it can linearly adjust the reactive power for improving the powerfactor.

[0030] Referring to FIG. 7, it is illustrated that the hybrid reactivepower compensation device 3 of the third embodiment is parallelconnected between the power system 1 and the load 4. The power system 1supplies an AC power to the load 4. The hybrid reactive powercompensation device 3 is used to supply the reactive power demanded bythe load 4. The hybrid reactive power compensation device 3 improves theinput power factor to be closed to unity. The hybrid reactive powercompensation device 3 includes a passive type reactive power compensator31 and an active type reactive power compensator 32 serially connectedthereto. The passive type reactive power compensator 31 may be athyristor switch assembly 310 and an AC power capacitor assembly 311serially connected thereto to form a Thyristor Switch Capacitor (TSC).In practical application, the hybrid reactive power compensation device3 can be operated with different step numbers of the AC power capacitor311 therein by means of switching the thyristor switch assembly 310 thataccomplishes rough tuning for adjusting reactive power. Moreover, it canadjust the reactive power for fine-tuning by means of the active typereactive power compensator 32 that improves the input power factor to beclosed to unity. The active type reactive power compensator 32 applies acontrol method of the first embodiment that generates the current withfundamental waveform. Consequently, the AC power capacitor assembly 311formed in the passive type reactive power compensator 31 can avoid thedestruction caused by the power resonance.

[0031] Although the invention has been described in detail withreference to its presently preferred embodiment, it will be understoodby one of ordinary skill in the art that various modifications can bemade without departing from the spirit and the scope of the invention,as set forth in the appended claims.

What is claimed is:
 1. A hybrid reactive power compensation deviceparallel-connected to a power system to provide reactive power tothereby improve the power factor, comprising: a passive type reactivepower compensator; and an active type reactive power compensatorserially connected to the passive type reactive power compensator;wherein the passive type reactive power compensator provides thereactive power so that power capacity of the active type reactive powercompensator is reduced; the active type reactive power compensator ofthe hybrid reactive power compensation device can supply the linearlyadjustable reactive power within a predetermined range; the active typereactive power compensator forces the current passing through the hybridreactive power compensation device to be sinusoidal, thereby avoidingthe destruction of passive type reactive power compensator caused by thepower resonance.
 2. The hybrid reactive power compensation device asdefined in claim 1, wherein the passive type reactive power compensatoris an AC power capacitor or a thyristor switching capacitor.
 3. Thehybrid reactive power compensation device as defined in claim 2, whereinthe passive type reactive power compensator is a thyristor switchingcapacitor, which is used to supply the adjustable reactive power forrough tuning.
 4. The hybrid reactive power compensation device asdefined in claim 1, wherein the active type reactive power compensatoris consisted of a power converter, a DC power capacitor, ahigh-frequency ripple filter and a controller.
 5. The hybrid reactivepower compensation device as defined in claim 1, wherein the active typereactive power compensator adopts the current mode control.
 6. Thehybrid reactive power compensation device as defined in claim 5, whereinthe current mode control of the active type reactive power compensatorprovides a first control signal and a second control signal consisted inreference signals, the reference signals and the output current of theactive type reactive power compensator are controlled by a controlcircuit so that the output current of the active type reactive powercompensator is consistent with the reference signal.
 7. The hybridreactive power compensation device as defined in claim 6, wherein thefirst control signal is adapted to accomplish a function for adjustingreactive power, the first control signal is a sinusoidal signal leadingwith the voltage signal of fundamental component of the power system by90 degrees since the active type reactive power compensator iscontrolled by the current mode control, which adjusts the amplitude ofthe first control signal to thereby provides with an linearly adjustablereactive power; the second control signal is adapted to regulate thevoltage of a DC capacitor of the active type reactive power compensator,the active type reactive power compensator must generate a fundamentalsinusoidal signal in phase with the voltage of power system so that theactive type reactive power compensator can absorb real power from apower system or generate real power to it, thereby regulating thevoltage of the DC capacitor of the active type reactive powercompensator.
 8. The hybrid reactive power compensation device as definedin claim 1, wherein the hybrid reactive power compensation device isparallel-connected to an automatic power factor regulator system, theautomatic power factor regulator system is able to adjust the reactivepower for rough tuning, and the hybrid reactive power compensationdevice can supply a sinusoidal current to linearly adjust the reactivepower for fine tuning that it can improve the input power factor to beclosed to unity, thereby reducing the capacity of the hybrid reactivepower compensation device.